Process for alkylation utilizing evaporative cooling



Dec. 29, 1953 D. H. PUTNEY 2,664,452

PROCESS FOR ALKYLATION UTILIZING EVAPORATIVE COOLING Filed NOV. 7, 1952 Patented Dec. 29, 1953 orties PROCESS FOR ALKYLATION UTILIZING EVAPORATIVE COOLENG David H. Putney, Kansas City,` M0., assigner to Stratford Engineering Corporation, Kansas City, Mo., a corporation of Delaware Application November 7, 1952, Serial No.` 319,244H

6 Claims. l Thisinvention relates tov improvements m' a processvfor the alkylation of hydrocarbons 1n the presence of a lliquid acid catalyst, and refers more particularly to a process in Which evaporatwe cooling of the hydrocarbons is utilized to reduce the temperature of the reactants prior to and.

during-the reaction step.

This. application is a continuation-impart of anlapplication Serial No. 196,282, filed November 17'J 1950, noW U. S. Patent No. 2,649,486.

Inthe prior parent application there was d1sclosed atWo-stage absorption type alley/lationv procedure,` and one of the advantages of such process was the reduction in the amount of normalparatiin diluents present in the alkylation 1 zone.A This reduction in the .amount of diluents not only promotedbetter quality alkylate, but also made it possible to utilize the hydrocarbon eiiiuent from the reaction step as the refrigeratinrg medium in the process.

In the process hereinafter described, a single stage alkylation operation is disclosed eliminating Athe absorption step. In thistype process the quantity of diluents present in the reaction cannot be used as a refrigerating medium without buildinq up a still larger quantity ot diluents' in the-v reaction cone.` alkylation unit Without ythe absorption step, but in` which hydrocarbon effluent isused as a re-v frigerant, the vapors separated from the hydrocarbon eiiluent may be ccmposedconservatively of 50% to 60% isobutane, 35% to 45% normal butano, and approximately 5% other hydrocarbons represented by lighter' fractions of thealkylate. The presence of this 35% to,45% normal lontane in the condensatereturnedto the reaction `zone not only does not contributeto the, establishment offproper alkylationV conditions,v but actu-ally has a detrimentaleifect thereon.W

Thusin the instant process, .the presence ofthe 35%. to 45% normal butane in itself hasra detrimentalveiect on the allzylate Quality and, in addition, the low percentage of isobutane in the refrigerant vapors, that is, 50% to 60%, makes it diflicult to build up a satisfactory iscbutanemoleiin ratio.

The use of the effluent as a refrieerant, therefore, is advantageous principally in those cases where-the hydrocarbons evaporated tl'nerefrorn` f arecricher is'olmtanel than the mixture ofthe fresh feed .plus isobutanetrecycle from' fractiona tieney There wouldfbe little advantage in using effluent refrigeration in a'case where the vapors" from the effluent vvheri.` condensed and. added;

For example, in atypical back' to the vfeedvvould decrease the percent or isobutane in the reactant mixture and increasel the amount of normal rparaffin diluents in the. total combined feed.

An object of the invention, therefore, is to provide a process wherein the eluent from the reaction stage is separated into a hydrocarbon .phasel and an acid phase, the hydrocarbon phasepassed to an evaporative cooler Where it is chilled by a reduction of pressure andthe chilled liquid recondensed and themselves chilled by evaporativecoolingprior to beingrecycled intothereaction stage is in many cases so high that vthe effluent 'Si step- Ay further object is toprovide a-process Wherein' the chilled hydrocarbons used as acooling medium' in the reaction` step are returnedand combined with. the hydrocarbon. phaser` sepa-v` rated from the eluent mixtureandA directed thence ,to the evaporative cooling step.

Other and further obiects will appear from the description which followsf In the .accompanying drawing` which forms a: part of the instant specincation and is to be read in conjunction therewith merels indicate like parts.

The single figure is aschematic flow diagram ofzone embodiment of the process.`

Referring to the drawing, at I0 is shown azreactor comprising a vertical vessel preferably' like reference nuequippe'd with an open-*ended circulating tube,v

shown in dotted lines at Ita,- and a propellerA II in the bottom ofthe tube driven by aA motorv I2 through gearing enclosed within the casinar I 3.

The propeller is mounted-on a vertical shaft I4 extending upwardly from the top'of the gear case Is. To simplify an understanding of the apparatus, .it will be described in conjunction with the process employedf Olenic hydrocarbonsare Vintroduced into the bottomof the reactor throughla feed line I5, while the isobutaner to ybe alkylated is supplied from rtank it by means vof pump I'l and line I8.

The: isobutane supplyline la joins pipe I5 so the mixture of isobutane and olefins is introduced into the bottom of the reactor through line I5. Fresh acid is supplied to the system through pipe I9 and is charged by means of pump 20 through pipe 2I into the bottom of the reactor. The olefinic and parainic hydrocarbons supplied through pipe I5 are combined with the acid and alkylation takes place while the mixture is rapidly agitated in the reaction vessel. The effluent-hydrocarbon-acid mixture is withdrawn from reactor I0 through pipe 22 and is discharged into an acid settler 23 beneath hood 23a, where the heavier acid phase is separated from the hydrocarbon phase. The acid is withdrawn from the bottom of the settler through pipe 24, and may be withdrawn from the system through pipe 24a or directed to the suction side of pump to be recycled to the reactor with or without fresh acid, as the operation requires. The hydrocarbon phase is withdrawn from the top of settler 23 through pipe 25 and is directed through line 26 to the evaporative cooler 21. The settler 23 is maintained under sufficient pressure to keep all hydrocarbon constituents of the mix in liquid phase. Back pressure valve 25a in line 25 accomplishes this purpose. Upon being introduced into the cooler, pressure upon the hydrocarbon phase is reduced and the volatile fractions are evaporated. The liquid hydrocarbons chilled by evaporative cooling accumulate in the vessel 21 from which they are drawn off through pipe 28. These chilled liquids are recycled by means of pump 23 and pipes 30, and 3! to the top of the reactor I0 where the liquid is discharged into the header of a tubular heat exchanger extending substantially throughout the length of the reaction vessel, but not shown in the drawing. These heat exchange tubes are located within the circulating tube Illa and serve to chill the reactant mixture while it is being intimately mixed and rapidly circulated by the propeller I I. No back pressure is imposed upon the cooling medium in the heat exchange tubes, the pressure being substantially the same as that existing in the evaporative cooler 21. The lighter hydrocarbon components are thus permitted to vaporize within the tubes as heat is picked up from the reaction zone. The cooling medium after circulation through the heat exchanger is discharged as a mixture of liquid and vapor through pipe 32, which. is connected into pipe 26 through which the hydrocarbon phase is withdrawn from the top of the acid settler 23. Thus it will be seen that the chilled hydrocarbons utilized as a cooling medium in the reactor I0 are combined with the hydrocarbons separated from the acid in the settler and returned to the evaporative cooling step 21 through pipe 26.

A portion of the cooling medium returned through pipe 36 to the reactor may be diverted through line 33 to cool the incoming olen feed by means of heat exchanger 34 interposed in feed line I 5. The portion of the cooling medium utilized for such purpose is then discharged through pipe 35 to neutralization and fractionation, diagrammatically shown at 36. The amount of cooling medium used for this purpose is governed by the liquid accumulation in cooler 21 through a liquid level control device 21a connected to valve 31 in pipe 33.

The volatile hydrocarbons separated from the liquid in cooler 21 are discharged through pipe 38 and are directed to compressor 39, after which they are cooled in condenser 40 and discharged through pipes 4| and 42 into the isobutane accumulator I6. To facilitate the cooling of the accumulated liquid in drum I6, a portion of the volatile material is removed through pipe 43, and passed to compressor 39a, then cooled in condenser 40a, and returned to the drum I6 through pipes 4I and 42. A diversion line 44 connected to pipe 4I permits the bleeding of a portion of the hydrocarbons from the system to a depropanizer included in fractionation, indicated diagrammatically at 36. The isobutane accumulator is also equipped with a fresh isobutane feed line 45 and an isobutane recycle feed line 46 for returning isobutane from fractionation into the isobutane accumulator I6. Connected into fractionation diacrammatically shown at 36, are discharge lines 41, 48, 49, and 56, through which are respectively withdrawn propane and lighter hydrocarbons, normal butane, rerun alkylate and alkylate bottoms.

Although the drawing shows evaporative cooler 21 located below the top of the tubular heat exchanger of the reaction vessel thus requiring a pump to circulate cooled hydrocarbons through the tubes, it is to be understood that Vessel 21 may to advantage be located above the top of the reaction vessel so that cooled hydrocarbon liquid from it will flow by gravity through the elements of the tubular heat exchanger within the reactor. The mixture of liquid and vapor then issuing from the tubular heat exchanger is carried back up to vessel 21 by the gas lift effect of the vapor portion of the stream. When cooler 21 is so located pump 29 can be omitted. In this case however a pump will be required to pump the hydrocarbon product through control valve 31, line 33, exchanger 34 and line 35 to neutralization and fractionation.

In order to realize the benefits of effluent refrigeration, it is unnecessary to completely eliminate the normal parafiins from the feed as is substantially accomplished in the two-stage absorption type alkylation described in my prior application. There are however many instances in refineries where an isobutane fraction is alkylated with olefins originating from catalytic cracking units or other sources where the amount of normal parafn diluents in the combined feed is very low, being of the order of 8% to 10% or lower. In `sorne cases when recycling isobutane from fractionation containing approximately 80% isobutane and 20% normal butane, in quantities sufficient to build up an isobutane to olefin ratio in the total feed of 4:1, the vapors leaving the effluent refrigerant system or evaporative cooler will show as high as to 85% isobutane, which is considerably greater than the percentage of isobutane in the total feed. In such cases efuent refrigeration works to great advantage not only in increased quality of alkylate, but also in reducing the requirements for fractionating recycle isobutane. The higher the percentage of normal paraflins such as normal butane and propane in the feed, the less attractive the effluent refrigeration system becomes. When the volumetric percentage of normal paraffin in the composite feed is of the order of 25%, the efuent refrigeration system has lost most of its advantages. Above 25% normal parains in the feed, it has little to offer as compared with closed cycle refrigeration. Approximately 30% normal parafns in the feed Would result in from 35% to 40% normal parains in the vapors evaporated from the evaporative cooler and sent to the compressors,

which, as suggested in my previous application, would ,be unprontable operation.

The instant process, then, eliminates the absorption step and is limited to the processing of feed stocks for which absorption is not required in order to realize a substantial portion of the benefits of effluent refrigeration. When absorption is not used, there is no necessity for chilling the acid to the low temperature required in the system where an absorber is used.

It should be noted that in this simpler embodiment of the alkylation method, the eiuent `from the settler is passed to an evaporative cooler which acts as a ash drum where the hydrocarbon eiiuent is self-refrigerated by evaporative cooling, and from which the chilled liouid is passed to the reactor as a cooling medium. A portion of this cooling medium may be employed to cool the incoming olefin feed and since the olefin feed is combined with the isobutane feed prior to their introduction into the re-V actor, this cooling effect is transmitted to the entire feed.

Also, in the alkylation process described, the content of the isobutane accumulator drum I6 is self-refrigerated by continuous evaporation of isobutane to the compressor 39a, after which it is licueed and returned to the accumulator. Since isobutane compressors are available for evapora-ting isobutane from the hydrocarbon effluent, these same compressors can be utilized to self-refrigerate a portion or all of the isobutane feed, recycle and refrigerant condensate. The cooling of the isobutane feed in this manner reduces the load on the reactor cooling coils and the cost of these coils. as a consequence, is reduced. It is possible for this process to cool the isobutane feed in drum i6 to a temperature even below that of the reaction temperature without reouiring subatrnospheric pressures at the suction of compressor 39a, assuming that the reaction is carried out Within the range of 45 F. to 70 F.

It should be noted also that evaporative refriveration cannot be applied to the olefin feed since a portion of the refrigerant vapors must be continuously bled olf to the depropanizer of the fractionation section in order that propane can be eliminated from the system. If the olefin feed were introduced into the isobutane accumulator it, olens would be lost to fractionation and reiected from the system without reaching the alkylation zone. Only in very rare cases are alkylation feed stocks entirely devoid of propane. This propane is evaporated from cooling vessel 2l', preferentially to theisobutane. The compressor condensate recovered from these hydrocarbons contains a higher percentage of propane than does the liquid bottoms taken from cooler 2l and it is. therefore, a portion of this lighter stream which should be diverted to the depropanizer, to prevent building up propane to a high value in the reaction or alkylation zone.

Thus it will be seen that under selected conditions Where the feed stream to the alkylation stage is relatively low in normal parafns, evaporative cooling may be employed eiiiciently and to great advantage.

Having thus described my invention, I claim:

l. In an alkylation process wherein isobutane and olens are contacted with a liquid acid catalyst in a reaction step, and a mixture of hydrocarbons containing less than 35% normal parains is withdrawn with the acid catalyst as eiluent from said reaction step, the improvement which comprises separating said effluent into a hydrocarbon phase and an acid phase in a separating step, reducing the pressure on the hydrocarbon phase in an evaporative cooling step to refrigerate the same, separating the vaporized hydrocarbons from the chilled liquid hydrocarbons and passing the latter in indirect heat exchange with the reactant mixture in the reaction step.

2. A process as in claim 1 wherein a portion of the chilled liquid hydrocarbons returned to cool the reaction mixture is brought into indirect heat exchange with the feed of reactants to said reaction step.

3. A process as in claim 1 wherein a portion of the chilled liquid hydrocarbons returned as a cooling medium to the reaction step is brought into indirect heat exchange with the feed of reactants to said reaction step, said portion thereafter diverted to a fractionation step, and controlling the amount of said diverted portion by the liquid level in the evaporative cooling step.

4. A process as in claim 1 wherein the chilled hydrocarbons returned from the evaporative cooling step and used as a chilling medium in the reaction step are passed from the reaction step back to the evaporative cooling step.

5. A process as in claim 1 wherein the vaporized hydrocarbons separated in the evaporative cooling step are compressed, condensed and recycled as a part of the feed to the reaction step.

6. A process as in claim 1 wherein the vaporized hydrocarbons separated in the evaporative cooling step are compressed, condensed, and a portion re-evaporated from the condensed body of liquid to reduce the temperature thereof prior to recycling as a part of the feed to the reaction step.

DAVID H. PUTNEY.

References Cited in thele of this patent UNITED STATES PATENTS Number Name Date 2,380,245 Keith July 10, 1945 2,389,604 Dauding Nov. 27, 1945 2,416,760 Lawler et al. Mar. 7, 1947 2,488,943 Shearer Nov. 22, 1949 

1. IN AN ALKYLATION PROCESS WHEREIN ISOBUTAN AND OLEFINS ARE CONTACTED WITH A LIQUID ACID CATALYST IN A REACTION STEP, AND A MIXTURE OF HYDROCARBONS CONTAINING LESS THAN 35% NORMAL PARAFFINS IS WITHDRAWN WITH THE ACID CATALYST AS EFFLUENT FROM SAID REACTION STEP, THE IMPROVEMENT WHICH COMPRISES SEPARATING SAID EFFLUENT INTO A 